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<div class="titlepage"><div><div><h3 class="title">
<a name="log.rationale.why_not_lazy_streaming"></a><a class="link" href="why_not_lazy_streaming.html" title="Why not using lazy streaming?">Why not using lazy
      streaming?</a>
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<p>
        One of the possible library implementations would be using lazy expressions
        to delay log record formatting. In essence, the expression:
      </p>
<pre class="programlisting"><span class="identifier">logger</span> <span class="special">&lt;&lt;</span> <span class="string">"Hello, world!"</span><span class="special">;</span>
</pre>
<p>
        would become a lambda-expression that is only invoked if the filtering is
        successful. Although this approach has advantages, it must be noted that
        lazy expression construction is not zero-cost in terms of performance, code
        size and compile times. The following expression:
      </p>
<pre class="programlisting"><span class="identifier">logger</span> <span class="special">&lt;&lt;</span> <span class="string">"Received packet from "</span> <span class="special">&lt;&lt;</span> <span class="identifier">ip</span> <span class="special">&lt;&lt;</span> <span class="string">" of "</span> <span class="special">&lt;&lt;</span> <span class="identifier">packet</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="string">" bytes"</span><span class="special">;</span>
</pre>
<p>
        would generate a considerable amount of code (proportional to the number
        of streaming operators) to be executed before filtering takes place. Another
        drawback is that the <code class="computeroutput"><span class="identifier">packet</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span></code>
        is always called, whether or not the record is actually written to the log.
        In order to delay this call, yet more scaffolding is needed, possibly involving
        <a href="https://www.boost.org/doc/libs/release/libs/bind/doc/html/bind.html" target="_top">Boost.Bind</a>,
        <a href="https://www.boost.org/doc/libs/release/doc/html/lambda.html" target="_top">Boost.Lambda</a>
        or <a href="https://www.boost.org/doc/libs/release/libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>.
        This complication is not acceptable for such a basic use case, like this.
      </p>
<p>
        Although lazy streaming is not provided by the library out of the box, nothing
        prevents developing it in a separate hierarchy of loggers. See the <a class="link" href="../extension/sources.html" title="Writing your own sources">Extending the library</a> section for
        more information.
      </p>
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<div class="copyright-footer">Copyright © 2007-2024 Andrey Semashev<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>).
      </p>
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